The objectives of the proposed research are to explore the closely linked functional relationships between membrane specialization, Na transport sites and the routes of transmembrane and transepithelial salt and water transport across reabsorptive and secretory epithelia. Particular attention is directed toward the localization of ouabain-sensitive Na transport sites (e.g., Na-K-ATPase) at the light and electron microscopic level in reabsorptive (e.g., kidney) and secretory epithelia (e.g., avian salt glands, teleost gill and operculum). The techniques employed are light microscopic autoradiography of 3H-ouabain binding sites in freeze dried tissues and ultrastructural localization of the ouabain-sensitive and K-dependent phosphatase component (K-NPPase) of the Na-K-ATPase enzyme complex. The localization of Na-K-ATPase in secretory epithelia will be related to membrane specialization and, in particular, to junctional morphology as it appears in thin sectioned and freeze fractured tissue. In addition, we will examine the relationship between the localization of ouabain-sensitive Na pump sites in a model secretory system, the avian salt gland, and ouabain-sensitive and insensitive ion fluxes in metacholine-stimulated and non-stimulated slices as they are revealed by established tracer techniques.